Hi, welcome to g o y, shrub stumps would be better dug out if you can manage it, as they could take several years to rot, and if you have applied stump killer, you will not be able to use the ground for anything else until all trace has gone, having said that, i you are unable to remove the stump, you can drill holes in the stump and any roots that you can get to, and apply s b k, cover and leave, but as I say it's a long process, Derek.